The present invention relates in general to semiconductor devices and, more particularly, to low frequency filter networks formed on semiconductor substrates.
Wireless communications devices typically operate using both radio frequency (RF) signals and lower frequency audio signals. For example, cellular telephones transmit RF carrier signals that operate at frequencies of six gigahertz or more and are modulated with audio frequency voice information. A microphone generates an audio frequency signal from the voice information which is amplified and used to modulate the RF carrier signal. Most wireless communications devices use a low pass filter at the microphone input to suppress ambient RF carrier signals that may be "picked up" or detected by the microphone in order to avoid degrading the performance of the communications device by noisy operation, loop instability, or other effects that reduce the quality of the modulating audio signal. To accomplish this function, the low pass filters have a passband in the audio range, i.e., less than about twenty kilohertz.
Presently, these audio filters are formed with discrete passive components because of the difficulty of forming the large component values that set the filters' low frequency passband. However, the discrete filters add a substantial fabrication cost to a wireless device. Integrated filters based on semiconductor technology have a lower cost but have not been practical because of the large die area needed to integrate audio frequency components while providing an adequate voltage capability.
Hence, there is a need for an integrated filter that provides a high level of frequency selectivity while maintaining a low manufacturing cost.
Prior art document
Prior art document
Prior art document
The above objectives are achieved by an integrated filter according to claim 1.
In the figures, elements having the same reference number have similar functionality.
Filter 10 is a low pass microphone line filter used to suppress RF components of input signal VIN from other circuitry of communications device 3 such as audio amplifier 9. That is, filter 10 passes the audio frequency components of input signal VIN while rejecting or attenuating RF components. The audio components are generated by microphone 4 from voice information, while the RF components are produced by, for example, incident electromagnetic waves generated by antenna 5 at the VXMIT carrier frequency. In the case of a cellular telephone, where microphone 4 is in close proximity to antenna 5, the RF components, if not attenuated or suppressed, can have an amplitude sufficient to overload audio amplifier 9 or to cause signal distortion, noise, instability, or other undesirable effects on the performance of communications device 3. Filter 10 has an output at a lead 65 for producing a filtered audio output signal VOUT. Filter 10 is specified to pass audio frequency components of VIN while attenuating RF component frequencies by a factor of at least thirty decibels at a frequency of six gigahertz. Hence, VOUT is substantially comprised of audio frequency components with few or no RF components.
Audio amplifier 9 amplifies output signal VOUT and produces an amplified audio signal VAUD. Oscillator 6 generates an RF oscillator signal VOSC at the desired carrier frequency of transmitter signal VXMIT. Modulator 8 modulates VOSC with VAUD and produces a modulated signal VMOD which is coupled to power stage 7 and amplified to produce transmitter signal VXMIT. In one embodiment, VXMIT has an RF carrier frequency of about six gigahertz.
Diodes 17-18 of ESD device 20 comprise back to back zener or avalanche diodes formed as junctions in a semiconductor substrate as described below. Diodes 17-18 are referred to as back to back diodes because their common cathode (or, alternatively, common anode) arrangement results in one of them being reverse biased regardless of the polarity of VIN. ESD device 20 dissipates electrostatic energy in the form of high voltage peaks of short duration which could damage sensitive system components. In one embodiment, ESD device 20 is formed to comply with International Electrotechnical Commission standard IEC61000-4-2 level four. In the embodiment of
Inductor 74 is formed as a planar spiral inductor to have a typical value in a range between 1-5 nanohenries. In one embodiment, inductor 74 is formed by patterning a standard metal interconnect layer.
Capacitors 21-22 are formed as trench capacitors connected as shown to respectively produce capacitances C21=C22=1.0 nanofarads, approximately, that modify the frequency response of VIN to produce filtered output signal VOUT. The trench design provides capacitors 21-22 with a low equivalent series resistance, and therefore a high quality factor, which results in a low impedance and high quality filtering function at RF frequencies.
Resistor 24 is formed as a thin film resistor, typically with a low parasitic substrate capacitance for enhanced filter performance. Resistor 24 cooperates with capacitors 21-22 to establish a characteristic frequency response for filter 10. In one embodiment, resistor 24 is formed with doped polysilicon having a concentration selected to produce the specified resistance value in a small die area while providing a high level of control to maintain the resistances within a specified tolerance. In one embodiment, the value of resistor 24 is controlled to within plus or minus ten percent. In one embodiment, resistor 24 has a resistance of about fifty ohms and a temperature coefficient of resistance approaching zero.
Clamp diode 27 is an avalanche diode that breaks down to limit the voltage swing at lead 65 to avoid overloading the input stage of amplifier 9. Accordingly, clamp diode 27 also provides an ESD protection function at lead 65. In one embodiment, clamp diode 27 is formed with a structure similar to that of either diode 17 or diode 18, and therefore has similar characteristics, i.e., a breakdown voltage of about 13.3 volts.
A base layer 30 is formed with semiconductor material and heavily doped to function as a low resistance ground plane for filter 10. In one example, base layer 30 has a doping concentration in a range between 1016 and 1021 atoms/centimeter3. According to the claimed invention, base layer 30 has a doping concentration in a range between 1018 and 1021 atoms/centimeter3. For example, base layer 30 may comprise monocrystalline silicon doped to provide a p-type conductivity and a doping concentration of about 2*1020 atoms/centimeter3. The low resistivity of base layer 30 provides an effective ground plane that attenuates parasitic signals that would otherwise propagate through base layer 30 along parasitic signal paths to produce crosstalk and degrade filter performance.
An epitaxial layer 31 is grown over base layer 30 and doped to have an n-type conductivity. Epitaxial layer 31 forms a junction with base layer 30 to comprise diode 18, so the doping concentration of epitaxial layer 31 is selected to provide a specified avalanche voltage for diode 18 such as, for example, 13.3 volts. Epitaxial layer 31 typically has a thickness in a range between two and ten micrometers. In one embodiment, epitaxial layer 31 is grown to a thickness of about 2.5 micrometers and a concentration of about 5*1017 atoms/centimeter3.
A layer 32 is formed over epitaxial layer 31 to have an n-type conductivity. A doped region 33 is formed by introducing p-type dopants from a surface 35 of substrate 11 to produce a junction that functions as diode 17. The doping concentrations of epitaxial layer 32 and doped region 33 are selected to provide a specified avalanche voltage for diode 17 such as, for example, 13.3 volts. In one embodiment, layer 32 is an epitaxial layer grown to a thickness of about three micrometers and a concentration of about 1*1017 atoms/centimeter3, and doped region 33 has a thickness of about one micrometer and a surface, concentration of about 6.0*1019 atoms/centimeter3. Alternatively, epitaxial layer 31 is grown to a thickness of about 5.5 micrometers and layer 32 is formed.by subjecting epitaxial layer 31 to a blanket p-type diffusion to reduce its effective concentration to set the breakdown voltage of diode 17 to the desired level. This diffusion step reduces the effective doping concentration of epitaxial layer 31 within a depth less than about three micrometers.
An isolation region or sinker 12 is formed as a ring around ESD device 20 with a p-type conductivity and a depth of about twenty micrometers to electrically isolate ESD device 20 from other components. Sinker 12 is diffused through epitaxial layers 31-32 to provide an external electrical contact to base layer 30 at surface 35, which is facilitated by adding a doped region 36 using the processing steps used to form doped region 33. Hence, doped region 36 has a p-type conductivity to electrically couple sinker 12 through doped region 36 to an interconnect trace connected to lead 62.
A channel stopper 34 is heavily doped to have an n-type conductivity and a depth of about three micrometers. Channel stopper 34 surrounds doped region 33 and prevents surface 35 from inverting to form a channel that would result in a conduction path from doped region 33 to base layer 20. In addition, channel stopper 34 increases ESD robustness of the device by ensuring the dissipation of lateral current flow injected during ESD event to avoid current filaments forming at surface 35.
A dielectric material is disposed on surface 35 and patterned and etched to produce dielectric regions 45. In one embodiment, dielectric regions 45 comprise silicon dioxide thermally grown to a thickness of about five hundred angstroms followed by a layer about one micrometer thick of deposited silicon dioxide.
Capacitor 21 is formed as a trench capacitor by etching semiconductor substrate 11 to a depth of about seven micrometers to form a plurality of trenches 40 within sinker 12 as shown. In an alternative embodiment, trench 40 comprises several rows of individual trenches or a single serpentine trench that extends along surface 35 and intersects the view plane multiple times as needed to produce C21=1.0 nanofarads of capacitance.
A dielectric material is formed to line inner surfaces of trench 40 to form a dielectric liner 38. In one embodiment, the dielectric material includes silicon nitride formed to a thickness of about four hundred angstroms.
A conductive material such as doped polysilicon is deposited and etched to form a conductive region 37 that fills trench 40 to function as a first electrode of capacitor 21 with sinker 12 functioning as a second electrode. Sinker 12 is coupled to lead 62 through shallow, heavily doped p-type contact region 36 that is formed with the processing steps used to form doped region 33. Capacitor 22 is formed in a similar fashion.
Clamp diode 27 is formed by the junction of base layer 30 and epitaxial layer 31 and isolated from other components by surrounding it with sinker 12 as shown. Hence, clamp diode 27 has a breakdown characteristic similar to that of diode 18 in ESD device 20.
A standard integrated circuit metal layer is deposited and etched to form bonding pads 60 and 61, along with interconnect traces. Inductor 74 is concurrently formed by patterning this standard integrated circuit metal layer. Other interconnect traces are represented schematically to simplify the figure.
Node 64 comprises a bonding structure shown as a metallic bump such as a solder bump or copper bump used for mounting filter 10 in a flip-chip fashion to a system circuit board (not shown). Alternatively, the bonding structure may comprise a wire bond or other suitable structure for providing external electrical and/or mechanical connections. The bonding structure has a parasitic inductance L64 of between about 0.05 and 0.1 nanohenries which produces an impedance or inductive reactance X64=2*Π*fc*L64 to input signal VIN, where fc is the RF carrier frequency of transmitter signal VXMIT. For example, if L64=0.1 nanohenries and fc=6.0 gigahertz, X64=2*Π*(6.0*109)*(0.1*10-9) has a value of about four ohms.
Output signal VOUT is provided at node 65 through a structure similar to that of node 64. The node 65 bonding structure has a parasitic inductance L65 whose value is similar to the value of L64.
P-type dopants are implanted through surface 35 and diffused into semiconductor substrate 11 to form well regions 51 and 54. In one embodiment, well regions 51 and 54 are formed to a depth of about fifteen micrometers. Well regions 51 and 54 typically are doped to a lower concentration than sinkers 12 but the same thermal cycle is used to diffuse well regions 51 and 54 and sinkers 12 into substrate 11. The lower concentration of well regions 51 and 54 results in their being shallower than sinkers 12.
N-type dopants are introduced into substrate 11 through openings in dielectric region 45 to form doped regions 52-53 within well region 51 and a doped region 56 within well region 54. Doped regions 52-53 form junctions with well region 51 that operate as back to back diodes 17-18, respectively, of ESD device 20. The doping concentrations of well region 51 and doped regions 52-53 are adjusted to provide a predefined breakdown voltage to meet the specified performance of ESD device 20. In one example, doped regions 52-53 are each formed with a rectangular shape to occupy an area of surface 35 which is about two hundred micrometers on a side. Note that because doped regions 52 and 53 are formed with the same processing steps the avalanche breakdown voltages and other performance parameters are symmetrical with respect to the polarity of the voltage on node 64.
Similarly, doped region 56 and well region 54 form a junction that comprises clamp diode 27.
In summary, the present invention provides an integrated filter circuit that achieves a specified frequency selectivity while utilizing integrated circuit technology to achieve a small physical size and a low manufacturing cost. A semiconductor substrate is formed with a trench that is lined with a dielectric layer. A conductive material is used to fill the trench to provide a capacitance that filters an input signal. Back to back diodes are formed in the substrate to avalanche when an electrostatic discharge voltage reaches a predetermined magnitude.
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.